Field of the Invention
The present invention relates to an image forming unit which includes a high-voltage power supply circuit capable of generating a bias voltage based on a voltage output from a common transformer used by a plurality of image forming units, and applying the generated bias voltage to the same type of processing components provided on the image forming units.
Description of the Related Art
There is known an image forming apparatus which adopts a tandem system for realizing full-color printing, for example. This type of image forming apparatus includes an image forming unit for each color of Y (yellow), M (magenta), C (cyan), and K (black). These image forming units are linearly arranged. In these units, the image forming unit for the color K is disposed closer to a secondary transfer area described below than the image forming units for the other colors.
In a full-color printing mode, a charging unit provided on each of the image forming units uniformly charges a surface of a corresponding rotating photosensitive drum at a predetermined potential in accordance with an applied charging bias voltage. An exposure unit generates optical beams for each color based on image data, and applies the generated optical beams to a corresponding charging area. As a result, an electrostatic latent image in each color is formed on the surface of the corresponding photosensitive drum. On the other hand, a developing roller contained in a developer provided on each of the image forming units rotates while receiving a developing bias voltage to supply toner in corresponding color to the corresponding electrostatic latent image. As a result, a toner image in each color is formed.
An intermediate transfer belt contacts each of the photosensitive drums on the downstream side with respect to the developer in the rotation direction of the photosensitive drum. A primary transfer roller for each color faces the photosensitive drum for the corresponding color with the intermediate transfer belt interposed between the primary transfer roller and the photosensitive drum. This structure produces a primary transfer area for each color between the intermediate transfer belt and the corresponding photosensitive drum. A primary transfer bias voltage is applied to each of the primary transfer rollers to transfer the toner image on each of the photosensitive drums to the same area of the rotating intermediate transfer belt in the corresponding primary transfer area. As a result, a full-color toner image is formed.
The intermediate transfer belt further contacts a secondary transfer roller on a predetermined side (such as the left side) of the photosensitive drum for the color K to form a secondary transfer area. A secondary transfer bias is applied to the secondary transfer roller. As a result, a full-color toner image carried on the intermediate transfer belt is transferred to a printing medium in the secondary transfer area. This printing medium passes through a known fixing device, and reaches a tray for discharge as a printed matter.
For example, there is known a conventional image forming apparatus which includes, in view of reduction of component and manufacture costs, a high-voltage power supply circuit for generating a developing bias voltage for all colors based on a voltage output from a common transformer used by developing rollers for the all colors provided as processing components (for example, see JP 2009-163030 A and JP 2002-162870 A).
According to this conventional image forming apparatus, however, electric damage (more specifically, film reduction) may be given to the photosensitive drums for Y, M, and C colors particularly at positions facing the developing rollers for Y, M, and C colors when the developing bias voltage is constantly supplied to the developing rollers for Y, M, and C colors from the high-voltage power supply circuit in a rotation stop state of the photosensitive drums for Y, M, and C colors in a monochrome printing mode. In other words, the lives of the respective photosensitive drums for Y, M, and C colors are influenced.
In addition, it is possible that the same transformer is used for all the colors to generate the charging bias voltage and the primary transfer bias voltage. When the charging bias voltage and the primary transfer bias voltage are constantly supplied to the charging units and the primary transfer rollers for Y, M, and C colors in the monochrome printing mode in this structure, uniform charging for the surfaces of the photosensitive drums for Y, M, and C colors becomes difficult in the subsequent color printing mode. In this case, image deterioration called an image memory may be caused.